Oligocene paleotopography and structural evolution of the Pah Rah Range, western Nevada: Implications for constraining slip on the right-lateral Warm Springs Valley fault in the northern Walker Lane.

摘要

Left-stepping, northwest-striking dextral faults in the northern Walker Lane accommodate ∼15 to 20% of the North American-Pacific plate motion. Tuff-filled Oligo-Miocene paleovalleys provide piercing lines from which to gauge offset across the strikeslip faults. To constrain offset across the northwest-striking dextral Warm Springs Valley fault, detailed mapping and structural analysis were carried out in the Pah Rah Range on a highly faulted ∼750 m thick, 31 to 23 Ma sequence of ash-flow tuffs that fill an ∼east-trending paleovalley carved in Mesozoic basement. The tuffs are locally interbedded with fluvial gravels and pinch out southward against a well-exposed paleovalley margin. Middle Miocene mafic lavas and conglomerate disconformably overlie the Oligo-Miocene tuffs.; A complex array of kinematically linked northwest-, northeast-, north-, and east-striking faults and northwest- to west-northwest-trending folds deform strata in the Pah Rah Range. A large, open, upright, gently northwest-plunging anticline near the range crest is probably extensional in origin and marks the intersection of oppositely dipping systems of northwest-striking normal faults and associated tilt domains. Proximal to the Warm Springs Valley fault, however, strata are deformed into tight, west-northwest-trending, gently plunging folds in both the Pah Rah Range and northern Curnow Range. These folds probably resulted from a combination of drag, clockwise rotation, and north-south shortening along the Warm Springs Valley fault. The southwest limb of the extensional anticline in the Pah Rah Range is overprinted by a west-northwest-trending fold near the Warm Springs Valley fault, suggesting that regional extension preceded the onset of strike-slip faulting. Paleomagnetic data indicate negligible vertical-axis rotation of the Pah Rah Range but significant clockwise rotation (>40o) within ∼2 km of the Warm Springs Valley fault.; The paleovalley in the Pah Rah Range may correlate with other paleovalleys across the Warm Springs Valley fault, either one on the northwest flank of Dogskin Mountain or another in the northern Curnow Range. A correlation with the Dogskin Mountain paleovalley implies ∼20 km of dextral offset on the Warm Springs Valley fault. However, some stratigraphic differences between these paleovalleys and the apparent termination of the Warm Springs Valley fault only 10 km southeast of the study area argue against such a correlation. Instead, similar stratigraphy and structures favor a correlation between paleovalleys in the Pah Rah Range and northern Curnow Range. Such a correlation indicates ∼4 to 8 km of dextral offset along the southeast part of the Warm Springs Valley fault.